Hydrostatic transmission

ABSTRACT

An improved hydrostatic transmission includes a hydraulic pump which is connected with a hydraulic motor by fluid conduits. A makeup pump is connected with these conduits to provide for the maintenance of a minimum fluid pressure in the conduits by replacing or making up any fluid which is exhausted or lost during operation of the hydrostatic transmission. A pair of pressure responsive valve assemblies control the flow of fluid from the makeup pump to the fluid conduits. To compensate for a transient or short duration decrease in the fluid pressure in one of the conduits, the associated valve assembly discharges fluid from the valve assembly into the conduit while blocking fluid flow from the makeup pump to the conduit. Toward the end of the short duration decrease in pressure, the fluid pressure in the conduit begins to rise back toward the desired minimum pressure and fluid is returned to the valve assembly from the conduit. In this manner a transient or short duration decrease in pressure in the conduit is compensated for without the addition of unnecessary makeup fluid.

Unite States Patent [72] Inventor Keith A. Boyd Mt. Clemens, Mich. [2]]App]. No. 803,161 [22] Filed Feb. 28, 1969 [45] Patented June 15, 1971[73] Assignee Eaton Yale 8L Towne Inc.

Cleveland, Ohio 54] HYDROSTATIC TRANSMISSION 17 Claims, 5 Drawing Figs.[52] US. Cl 60/53 [51] lnt.C| FlSb 15/18 [50] Field of Search ..60/53 B,53, r 53 A, 52; 103/203 56] References Cited UNITED STATES PATENTS3,003,309 10/1961 Bowers 60/53 3,300,969 l/l967 Barden 60/52 3,376,7034/1968 Buczynski 60/53 Primary ExaminerEdgar W. Geoghegan Allorney-Yountand Tarolli ABSTRACT: An improved hydrostatic transmission includes ahydraulic pump which is connected with a hydraulic motor by fluidconduits. A makeup pump is connected with these conduits to provide forthe maintenance of a minimum fluid pressure in the conduits by replacingor making up any fluid which is exhausted or lost during operation ofthe hydrostatic transmission. A pair of pressure responsive valveassemblies control the flow of fluid from the makeup pump to the fluidconduits. To compensate for a transient or short duration decrease inthe fluid pressure in one of the conduits, the associated valve assemblydischarges fluid from the valve assembly into the conduit while blockingfluid flow from the makeup pump to the conduit. Toward the end of theshort duration decrease in pressure, the fluid pressure in the conduitbegins to rise back toward the desired minimum pressure and fluid isreturned to the valve assembly from the conduit. In this manner atransient or short duration decrease in pressure in the conduit iscompensated for without the addition of unnecessary makeup fluid.

IiilYDROSTATlC TRANSMISSION This invention relates generally to ahydrostatic transmission and ore particularly to a means forcompensating for transient pressure variations in a fluid circuit of ahydrostatic transmission without the additions of unnecessary makeupfluid.

A known hydrostatic transmission includes a hydraulic pump and motorwhich are innerconnected by a closed loop fluid circuit. A makeup pumpis connected with both sides of the fluid circuit to replace or makeupfluid which may be lost through an exhausting of the circuit to areservoir through a high pressure relief valve or due to fluid leadagefrom the circuit during operating of the hydrostatic transmission. Aknown hydrostatic transmission having such a construction is illustratedin U.S. Pat. No. 3,376,703 to Buczynski.

While these known hydrostatic transmissions have been generallysatisfactory, difficulty has bee encountered due to erratic operation ofthe fluid motor during low speed and low torque load conditions. Thiserratic operation of the hydrostatic transmission is believed to becaused by intermittent and unnecessary operation of a check valve forcontrolling the flow of makeup fluid to the high pressure side of theclosed loop" fluid circuit interconnecting the pump and motor. Thisintermittent operation operation of the check valve is believed to bethe result of transient or short duration decreases in the fluidpressure in the high pressure side of the closed loop circuit. It istheorized that the short duration decreases in pressure are due toinstantaneous variations in the displacement of the fluid motor duringits operation.

Accordingly, it is an object of this invention to provided a new andimproved hydrostatic transmission which is capable of relatively smoothoperation during low speed and low torque load conditions.

Another object of this invention is to provide a new and improvedhydrostatic transmission having a pump and motor interconnected by afluid conduit with means to compensate for instantaneous variations inthe displacement of the motor without the addition of unnecessary makeupfluid to the fluid circuit between the pump and motor.

Another object of this invention is to provide a new and improvedhydrostatic transmission having a pump and motor interconnected by afluid circuit wherein an assembly responds to long duration decreases influid pressure in the circuit by enabling necessary makeup fluid to beadded to the fluid circuit and compensates for transient or shortduration decreases in fluid pressure in the circuit without addingunnecessary makeup fluid to the fluid circuit.

Another object of this invention is to provide a new and improvedhydrostatic transmission having a valve means for controlling theaddition of fluid to a fluid circuit between the pump and motor of thehydrostatic transmission wherein the valve means compensates for shortduration decreases in fluid pressure in the circuit by discharging fluidfrom the valve means during the short duration decrease in pressure andby accepting fluid from the circuit as the pressure in the circuitincreases.

These and other objects and features of the invention will become moreapparent upon a consideration of the following description taken inconnection with the accompanying drawings wherein:

FIG. 1 is a schematic illustration of a hydrostatic transmissionconstructed in accordance with the present invention;

FIG. 2 is a schematic illustration of a valve assembly for controllingthe flow of makeup fluid to a fluid circuit of the hydrostatictransmission and for compensating for transient or short durationdecreases in fluid pressure in the fluid circuit, the valve assemblybeing shown in the closed condition;

H6. 3 is a schematic illustration of the valve assembly in FIG. 2 in anintermediate condition wherein the valve assembly is blocking a flow ofmakeup fluid to the fluid circuit and is compensating for a shortduration decrease in the fluid pressure of the fluid circuit;

FIG. 4 is a schematic illustration of the valve assembly of FIGS. 2 and3 in an open condition in which makeup fluid is being added to the fluidcircuit; and

FIG. 5 is a schematic illustration of a second embodiment of theinvention.

The present invention relates to a hydrostatic transmission having ahydraulic pump and motor which are interconnected by a fluid circuit. Amakeup pump is connected with this circuit to replace fluid lost fromthe circuit due to leaks or exhausting of fluid to a reservoir through ahigh pressure relief valve. A valve assembly controls the addition ofmakeup fluid to the fluid circuit and is operable to an open conditionin response to only relatively long duration decreases in the fluidpressure in the circuit. The valve assembly compensates for shortduration decreases in the fluid pressure in the circuit by dischargingfluid from the valve assembly into the fluid circuit. As a shortduration decrease in pressure ends and the pressure in the fluid circuitincreases, the fluid which was added to the circuit from the valveassembly is no longer needed and the valve assembly accepts fluid fromthe circuit.

A hydrostatic transmission 10 embodying the present invention includes areversible hydraulic pump 12 which is connected with a reversiblehydraulic motor 14 by a closed loop" fluid circuit 16. During operationof the hydrostatic transmission 10, a carrier or rotor 20 of the pump 12is rotated by an engine or other suitable source of power to pump fluidunder pressure through a conduit 24' to the motor 14. This relativelyhigh pressure fluid causes the ball piston type motor 14 to operate in aknown manner to drive a vehicle or perform other operations. Relativelylow pressure fluid is conducted from the motor 14 through a conduit 26back to the pump 12 for recirculation.

The speed of operation of the motor 14 is controlled by means of a shiftlever 30 which is operable to vary the effective displacement of thepump 12 and the rate of fluid flow through the conduit 24. To accomplishthis, movement of the shift lever 30 moves a cam ring 32 to vary thestroke of ball type pistons 34 relative to the carrier or rotor 20 whichhas cylinders 38 in which the pistons 34 are slidably mounted. As iswell known to those skilled in the art, the direction of operation of hemotor 14 is reversed by reversing the direction of operation of the pump12. This results in relatively high pressure fluid being pumped throughthe conduit 26 to the motor 14 and returned to the pump 12 from themotor 14 through the conduit 24.

During operation of the hydrostatic transmission 10 it may be necessaryto exhaust the fluid circuit 16 through a high pressure relief valve(not shown) to a suitable low pressure reservoir. In addition, fluid maybe lost from the circuit 16 due to leakage and other causes. A makeuppump 44 is operable to replace exhausted or lost fluid by pumping fluidfrom a reservoir 38 through a conduit 50 and valve assembly 52 to theconduit 26 on the low pressure side of the fluid circuit 16. When thedirection of operation of the pump 12 is reversed, the makeup pump 44pumps fluid through the conduit 50 and a valve assembly 54 to what isthen the low pressure fluid conduit 24. It should be noted that thevalve assemblies 52 and 54 are of the nonreturn or check type to preventhigh pressure fluid from flowing from either the conduit 24 or 26 to theconduit 50.

During operation of the hydrostatic transmission 10, the fluid pressurein the high pressure fluid conduit of the circuit 16 tends to fluctuateor vary even though the pump 12 is operated at a constant speed. Thisvariation in fluid pressure in the high pressure fluid conduit appearsto be due to variations in the instantaneous displacement of the motor14. Variations in the instantaneous displacement of the motor 14 resultfrom rotation of a rotor 56 relative to a housing 48 of the motor 14.This rotation of the rotor 56 causes pistons 60 to move axially alongcylinders 62 formed in the rotor 56.

When the motor 14 is being operated in a clockwise direction (as viewedin FIG. l), the high pressure fluid conduit in the fluid circuit 16 isthe conduit 24. However, under low speed and low torque load conditions,the average fluid pressure in the conduit 24 is relatively low.Transient or short duration variations in fluid flow and pressure in theconduit 24 can occur even though the pump 12 is operated at a constantspeed. It is believed that these short duration variations in flow andpressure in the fluid conduit 24 are caused by instantaneous variationsin the displacement of the motor 14 and can result in the fluid pressurein conduit 50 being slightly greater for a short time than the fluidpressure in the conduit 24. The valve assembly 54 then tends to openunder the influence of the differential in fluid pressure between theconduits 24 and 50.

If the valve assembly 54 is allowed to operate to the open conditionunder the influence of transient variations in the fluid pressure in theconduit 24, makeup fluid flows from the conduit 50 to the conduit 24under the influence of the pump 44. However, the addition of makeupfluid to the conduit 24 results in an excess amount of fluid beingcontained within the conduit once the transient or short term decreasein pressure has passed due to continued operation of the motor 14.

To minimize erratic operation of the motor 14 due to instantaneousvariations in the displacement of the motor or other causes even thoughthe pump 12 may be driven at a constant speed, the valve assembly 54compensates for transient or short duration changes in fluid pressure inthe conduit 24 without adding unnecessary makeup fluid to the fluidcircuit 16. To accomplish this, the valve assembly 54 responds to thepresence of a transient or short duration depression in the fluidpressure in the conduit 24 by discharging fluid contained within thevalve assembly into the conduit 24. As the transient depression in thepressure passes, a substantially similar amount of fluid is returned tothe valve assembly 54 from the conduit 24. Since the amount of fluidreturned to the valve assembly 54 is substantially equal to the amountof fluid previously discharged from the valve assembly, the occurrenceof a transient variation in fluid pressure in the conduit 24 iscompensated for without the addition of unnecessary makeup fluid.

During a transient depression in fluid pressure in the conduit 24, thevalve assembly 54 blocks the flow of makeup fluid from the conduit 50 tothe conduit 24. However when the fluid is lost or exhausted from thefluid circuit 16, makeup fluid must be added to the fluid circuit.Therefore, the valve assembly 54 is operated to an open condition inresponse to a relatively long duration decrease in the fluid pressure inthe conduit 24. This enables makeup fluid to be pumped into the conduit24 to replace any fluid which may have been lost or exhausted duringoperation of the hydrostatic transmission 10.

The valve assembly 54 (see FIG. 2) includes a housing or body 64 havinga chamber 66 in which a ball type valve element 68 is located. Thechamber 66 is connected in fluid communication with the fluid conduit 24so that one side of the valve element 68 is exposed to the fluidpressure in the conduit 24. When the relatively high pressure fluid isbeing conducted through the conduit 24, the valve element 68 is pressedagainst an annular valve seat 76 to seal an inlet opening 76 to thechamber 66. This prevents high pressure fluid from flowing into themakeup conduit 50 from the conduit 24.

Upon the occurrence of a transient or short duration decrease in thefluid pressure in the conduit 24 to a pressure below the fluid pressurein the conduit 56, fluid is discharged from the valve assembly 54 intothe conduit 24. To provide for this discharging of fluid, the valveelement 68 is moved axially along a first cylindrical stage or portion80 of the valve chamber 66 by the higher pressure of the makeup fluid inthe conduit 50. As the valve element 68 moves axially along the inletportion or stage 80 of the chamber 66, the size of an outlet cavity 84formed between the valve element 68 and an outlet opening 88 in thehousing 64 is decreased from the relatively large volume of FIG. 2 tothe relatively small volume of FIG. 3. This decrease in the volume ofthe outlet cavity 84 results in fluid being discharged or expelled fromthe chamber 66 into the conduit 24. The fluid which is discharged fromthe chamber 66 into the conduit 24 tends to offset or compensate for thetransient depression in fluid pressure in the conduit 24 so that therate of operation of the motor 14 remains substantially constant.

While the outlet cavity 84 is being decreased in size by the movement ofthe valve element 68 from the closed position of FIG. 2 to theintermediate position of FIG. 3, an inlet cavity 93 (FIG. 3) is formednd expanded between the valve element 68 and inlet 76. This enablesmakeup fluid to flow into the valve housing or body 64 from the conduit50. However, the valve element 68 cooperates with the generallycylindrical interior surface 94 of the inlet stage or portion of thechamber 66 to block the flow of what is now relatively high pressuremakeup fluid from the conduit 50 to the conduit 24. Accordingly, thevalve element 68 has a diameter which is only slightly smaller than thediameter of the generally cylindrical surface 94 to thereby enable thevalve element 68 to move freely along the cylindrical surface 94 and atthe same time enable the valve element to block a flow of fluid from theinlet cavity 93 to the outlet cavity 84.

By the time the valve element 68 reaches the intermediate position shownin FIG. 3, continued operation of the motor 14 causes the fluid pressurein the conduit 24 to be increased. Since the fluid pressure in theconduit 24 has started to increase, the fluid discharged from the valveassembly 54 is no longer required to compensate for the short durationdecrease in fluid pressure in the conduit. Therefore, the valve assembly54 accepts from the conduit 24 a volume of fluid which is substantiallyequal to the volume of fluid previously discharged into the conduit.

To provide for acceptance of fluid from the conduit 24 by the valveassembly 54, the previous axial movement of the valve element 68 isreversed and the valve element is again pressed into sealing engagementagainst the valve seat 74 (see FIG. 2). During this movement of theelement 68 from the intermediate position of FIG. 3 to the closedcondition of FIG. 2, the outlet cavity is expanded to enable fluid toflow into the valve assembly 54 from the conduit 24. In addition, theinlet cavity 93 is decreased in size to expel the makeup fluid containedtherein into the fluid conduit 50. It should be noted that the axialextent of the inlet stage portion 80 of the chamber 66 is long enough sothat previously described transient variations in the fluid pressure inthe conduit 24 due to instantaneous variations in the displacement ofthe motor 14 are of insufficient duration to enable the valve element 68to move from the inlet portion or stage 80 of the valve assembly to anoutlet portion or stage 98 of the valve assembly.

After the fluid circuit has been exhausted when the direction ofoperation of the pump 12 is reversed, the conduit 24 conducts relativelylow pressure fluid. When it is necessary to add makeup fluid to therelatively low pressure fluid flowing through the conduit 24, thepressure of fluid in the conduit is below the pressure of the fluid inthe makeup conduit 50. Therefore, the valve element 68 is moved out ofsealing engagement with the valve seat 74 to the intermediate positionshown in FIG. 3. Since the decrease or depression of the fluid pressurein the conduit 24 lasts for a relatively long period of time, the valveelement 64 moves axially past the intermediate position of FIG. 3 to theopen position of FIG. 4.

In the open position of FIG. 4, the valve element 68 is located in theoutlet stage or portion 98 of the valve chamber 66. The outlet portion98 has a generally cylindrical surface 102 with a diameter which issubstantially greater than the diameter of the valve element 68.Therefore, when the valve element 68 is in the open position, makeupfluid flows from the conduit 50 through the inlet portion 80 of thechamber 66, around the valve element 68 and through the outlet portion98 of the chamber 66 to the conduit 24. It should be noted that suitableretaining elements 106 are provided at the outlet 88 of the camber 66 toretain the valve element 68 in the housing 64.

When exhausted fluid is replaced or the direction of rotation of thepump 12 is reversed, the valve element 68 is again exposed to arelatively high pressure. This high pressure moves the valve element 68axially from the open position of FIG. 4 to the closed position of FIG.2. Movement of the valve element or ball 68 from the open position tothe closed position is facilitated by the provision of a tapered guidesurface 110 between the inlet portion 80 and outlet portion 98 of thechamber 66.

The valve assembly 52 is identical in construction to the valve assembly54 an responds to the presence of both transient and long durationdepression in the pressure in the conduit 26 in the same manner as thevalve assembly 54. The valve assembly 52 includes a valve element 114(FIG. I) which is located in a housing 116 which defines a valve chamber117. The valve chamber 117 has a relatively small diameter inlet cavity118 and a relatively large diameter outlet cavity 120. The valve element114 is moved to an intermediate position similar to the intermediateposition illustrated in FIG. 3 for the valve element 68 when the fluidpressure in the conduit 26 is decreased to a pressure below the fluidpressure in the makeup conduit 50. If the decrease in fluid pressure inthe conduit 26 is transient or of short dura tion, the valve element 114returns to a closed condition similar to that illustrated in FIG. 2 forthe valve element 68. However, if the decrease in fluid pressure in theconduit 26 is of a relatively long duration condition, the valve element114 is moved to the open condition of FIG. 1 to enable makeup fluid tobe conducted from the conduit 50 through the valve assembly 52 to theconduit 26.

A second embodiment of the invention is illustrated in FIG. 5. Since theconstruction and mode of operation of this embodiment of the inventionis substantially the same as previously described in connection withFIGS. 1 to 4, numerals similar to those used to designate the elementsof FIGS. 1-4 will be used to designate the elements of FIG. 5. To avoidconfusion, the suffix letter A will be added to the numerals associatedwith FIG. 5.

In the embodiment of the invention illustrated in FIG. 5, a pair ofvalve assemblies 520 and 544 control the flow of fluid from the pump Mato the conduits 26a and 24a. The valve assemblies 52a and 540 areconnected by a conduit 50a with a makeup pump 44a. The valve assemblies52a and 54a compensate for transient or short duration decreases in thefluid pressure in the conduits 24a and 26a in much the same mannerpreviously explained in connection with the valve assemblies 52 and 54.

The valve assembly 54a includes a valve element 68a located in a chamber66a in a housing 640. The valve element 68a is movable between the opencondition illustrated in FIG. 5 and a closed condition in which thevalve element 68a sealingly engages a valve seat 74a which circumscribesan inlet 76a to the housing or body 640. The chamber 66a is divided intotwo stages or portions, that is a relatively small diameter inlet stageor portion 80a and a relatively large diameter outlet stage or portion98a. When the valve element 68a is in the open position of FIG. 5, fluidcan flow from the makeup pump Ma through the conduit 50a and inletportion 80a of the valve assembly 54a. The fluid then flows around thevalve element 68a and through the outlet portion 98a of the valveassembly 54a into the conduit 24a which is conducting relatively lowpressure fluid to which makeup fluid must be added.

The valve assembly 52a includes a valve element 114a which is located ina body or housing 116a defining a valve chamber 1117a. The chamber 117ahas a relatively small diameter first or inlet portion 118a andrelatively large diameter second or outlet portion 120a. The valveelement 114a is connected with the valve element 68a by a connectormember 160 which extends through the conduit 500 between the valveassemblies 52a and 54a.

The connector member 140 interconnects the valve elements 114a and 68aso that the valve assembly 52a is in the closed condition of FIG. 5 whenthe valve assembly 54a is in the open condition. Similarly, when thevalve assembly 540 is in a closed condition, the connector member 140moves the valve element 114a to the open condition. It should be notedthat the valve assemblies 52a and 54a compensate for transient or shortduration decreases or depressions in the relatively high fluid pressurein the conduits 26a and 24a by discharging fluid from the valveassemblies into the conduits. This is accomplished in the case of theconduit 26a by movement of the valve element 114a outwardly from aclosed position illustrated in FIG. 5 to an intermediate position,similar to that illustrated in FIG. 3 for the valve assembly 54. At theend of the short duration increase in pressure, fluid will flow form theconduit 26a into the valve assembly 52a is that there is no neteffective addition of fluid to the conduit 26a in response to thetransient or short duration depression in the fluid pressure in theconduit 26a. Of course, when the conduit 24a is conducting relativelyhigh pressure fluid and the conduit 26a connecting low pressure fluidthe valve assembly 54a responds in the same manner to a transient orshort duration depression or decrease in the pressure in the conduit24a.

In view of the foregoing description, it can be seen that each of thevalve assemblies 52 and 54 compensates for short duration or transientdecreases in the fluid pressure in the fluid circuit 16 between the pumpand motor of a hydrostatic transmission by discharging fluid from thevalve assembly into the fluid circuit during the transient decrease influid pressure. At the end of the short duration or transient decreasein fluid pressure, the fluid flows from the fluid circuit 16 into thevalve assembly so that there is a substantially zero net addition offluid to the fluid circuit. In this manner each of the valve assemblies52 and 54 compensate for short duration decreases in the fluid pressurein the circuit 16 without adding unnecessary makeup fluid to thecircuit. Due to this compensation for short duration or transientdecreases in the fluid pressure in the fluid circuit 16 between the pump12 and motor 14, the operation of the motor is relatively smooth eventhrough the instantaneous displacement of the motor varies. Of course,the valve assembles 52 and 54 compensate for transient variations in thefluid pressure in the fluid circuit 16 which are caused by sources otherthan instantaneous variations in the displacement of the motor M.

Having described my invention, I claim:

1. A hydrostatic transmission assembly comprising a fluid pump, a fluidmotor, first fluid conduit means for conducting fluid between said pumpand motor upon operation of said pump to thereby operate said motor,second fluid conduit means for connecting said first fluid conduit meansin fluid communication with a source of fluid, and valve means connectedin fluid communication with said first and second fluid conduit meansand operable between open and closed conditions under the influence ofpressure differentials of relatively long duration between said firstand second fluid conduit means to thereby control fluid flow betweensaid first and second fluid conduit means, said valve means includingmeans for discharging fluid from said valve means into said first fluidconduit means under the influence of a relatively short durationdecrease in the pressure in said first fluid conduit means to a pressurebelow the pressure in said second fluid conduit means and for acceptingfluid from said first fluid conduit means at the end of the relativelyshort duration decrease in the pressure in said first fluid conduitmeans to thereby tend to minimize the effect of the relatively shortduration decrease in the fluid pressure in said first fluid conduitmeans.

2. A hydrostatic transmission assembly as set forth in claim 1 whereinsaid valve means includes a valve body defining a chamber connected influid communication with said first and second fluid conduit means and amovable valve element located in said chamber, said chamber including afirst portion having a cross-sectional area which is substantially thesame as the cross-sectional area of said valve element and a secondportion having a cross-sectional area which is substantially larger thanthe cross-sectional area of said valve element, said first portion ofsaid chamber and said valve element cooperating to block a flow of fluidbetween said first and second fluid conduit means and to form a cavitywhich is contractable and expansible by movement of :said valve elementrelative to said valve body to provide said means for discharging andaccepting fluid.

3. A hydrostatic transmission as set forth in claim l wherein said meansfor discharging and accepting fluid includes cavity means which iscontractable from a first condition to a second condition to dischargefluid from said valve means into said first fluid conduit means and isexpandable from said second condition to said first condition to acceptfluid from said first fluid conduit means.

4. A hydrostatic transmission assembly as set forth in claim 1 whereinsaid valve means includes a valve body defining a chamber having a firstportion of a relatively large cross-sectional area, and a valve elementmovable in said first portion of said chamber to provide for saiddischarging and accepting of fluid, said valve element being movablefrom said first portion to said second portion of said chamber toprovide for operation of said valve means from said closed condition tosaid open condition.

5. A hydrostatic transmission as set forth in claim ll wherein saidvalve means includes a valve body defining a chamber having an inletconnected in fluid communication with said second fluid conduit meansand an outlet connected in fluid communication with said first fluidconduit means, a valve element movable in said chamber, said means fordischarging and accepting fluid including an expansible cavity formed ina portion of said chamber by said valve element and connected in fluidcommunication with said outlet, said valve element being movable in afirst direction upon the occurrence of a relatively short durationdecrease in the pressure in said first fluid conduit means to a pressurebelow the pressure in said second fluid conduit means to contract saidcavity and discharge fluid from said valve body, said valve elementbeing movable in a second direction opposite from said first directionat the end of the relatively short duration decrease in pressure toexpand said cavity and accept fluid from said first fluid conduit means.

6. A hydrostatic transmission assembly comprising a fluid pump, a fluidmotor, first fluid conduit means for conducting fluid between said pumpand motor upon operation of said pump to thereby operate said motor,second fluid conduit means for connecting said first fluid conduit meansin fluid communication with a source of fluid, and valve means connectedin fluid communication with said first and second fluid conduit meansfor controlling fluid flow between said first and second fluid conduitmeans, said valve means including a body connected in fluidcommunication with said first and second fluid conduit means and meansresponsive to a short duration decrease in the fluid pressure in saidfirst fluid conduit means to a fluid pressure below the fluid pressurein said second fluid conduit means for blocking fluid flow from saidsecond fluid conduit means into said first fluid conduit means anddischarging fluid from said body into said first fluid conduit meansduring the short duration decrease in fluid pressure in said first fluidconduit means and for blocking fluid flow from said first fluid conduitmeans into said second fluid conduit means and accepting fluid from saidfirst fluid conduit means at the end of the short duration decrease inthe fluid pressure in said first fluid conduit means to thereby at leastpartially compensate for the short duration decrease in fluid pressurein said first fluid conduit means, said means responsive to a shortduration decrease in the fluid pressure in said first fluid conduitmeans also being responsive to a relatively long duration decrease inthe fluid pressure in said first fluid conduit means to a fluid pressurebelow the fluid pressure in said second fluid conduit means to enablefluid to flow from said second fluid conduit means to said first fluidconduit means.

7. A hydrostatic transmission assembly as set forth in claim 6 whereinsaid means responsive to a short duration decrease in the fluid pressurein said first fluid conduit means includes a movable valve element forblocking fluid flow between said first and second fluid conduit means,said valve element being located in a chamber defined by said body andcooperating with said body to define a cavity in said chamber, saidvalve element being movable in a first direction under the influence ofthe pressure differential between said first and second fluid conduitmeans during the short duration decrease in fluid pressure in said firstfluid conduit means to contract said cavity and thereby discharge fluidfrom said vicinity into said first fluid conduit means, said valveelement being movable in a second direction opposite to said firstdirection under the influence of a pressure differential between thefirst and second fluid conduit means to expand said cavity and to accepttherein fluid from said first fluid conduit means at the end of theshort duration decrease in fluid pressure in said first fluid conduitmeans, said valve element cooperating with said valve body during theshort duration decrease in fluid pressure in said first fluid conduitmeans to block fluid flow between said first and second fluid conduitmeans.

8. A hydrostatic transmission assembly as set forth in claim 7 whereinsaid valve element is movable for a relatively large distance in saidfirst direction under the influence of a relatively long durationdecrease in the fluid pressure in said first fluid conduit means to afluid pressure below the fluid pressure in said second fluid conduitmeans to move said valve element to a position in which it enables fluidto flow from said second fluid conduit means to said first fluid conduitmeans.

9. A hydrostatic transmission assembly as set forth in claim 7 whereinsaid valve element cooperates with said body to define another cavity insaid chamber, said other cavity being connected in fluid communicationwith said second fluid conduit means and being expanded by movement ofsaid valve element in said first direction during the sort durationdecrease in fluid pressure in said first fluid conduit means to acceptin said other cavity fluid from said second fluid conduit means duringthe short duration decrease in fluid pressure in said first fluidconduit means, said other cavity being contracted by movement of saidvalve element in said second duration at the end of the short durationdecrease in fluid pressure in said first fluid conduit means to therebydischarge accepted fluid from said second fluid conduit means back intosaid second fluid conduit means.

it). A hydrostatic transmission assembly comprising a fluid pump, afluid motor, first fluid conduit mean for conducting fluid between saidpump and motor, said motor means having an inherent displacementvariation during low speed operation resulting in a short durationpressure decrease in said first fluid conduit means, second fluidconduit means for connecting said first fluid conduit means in fluidcommunication with source of fluid, and compensating means including abody connected in fluid communication with said first and second fluidconduit means, said compensating means being responsive to the shortduration decrease in the fluid pressure in said first fluid conduitmeans to a fluid pressure below the fluid pressure in said second fluidconduit means for discharging fluid from said body into said first fluidconduit means during the short duration decrease in fluid pressure insaid first fluid conduit means and for accepting fluid from said firstfluid conduit means at the end of the short duration decrease in thefluid pressure in said first fluid conduit means to thereby at leastpartially compensate for the short duration decrease in fluid pressurein said first fluid conduit means, said compensating means including amovable element located in a chamber defined by said body andcooperating with said body to define a cavity in said chamber, saidelement being movable in a first direction under the influence of theshort duration decrease in fluid pressure in said first fluid conduitmeans to contract said cavity and thereby discharge fluid from saidcavity into said first fluid conduit means, said element being movablein a second direction opposite to said first direction under theinfluence of the fluid pressure in said first fluid conduit means toexpand said cavity and to accept therein fluid from said first fluidconduit means at the end of the short duration decrease in fluidpressure in said first fluid conduit means.

I l. A hydrostatic transmission assembly comprising a reversible fluidpump, a reversible fluid motor. first fluid conduit means for conductinghigh pressure fluid from said pump to said motor during operation ofsaid pump in one direction and for conducting relatively low pressurefluid from said motor to said pump during operation of said pump in anopposite direction, second fluid conduit means for conducting relatively low pressure fluid from said motor to said pump during operation ofsaid pump in said one direction and for conducting relatively highpressure fluid from said pump to said motor during operation of saidpump in said opposite direction, third fluid conduit means forconnecting said first and second fluid conduit means in fluidcommunication with a relatively low pressure source of fluid, firstvalve means connected in fluid communication with said first and thirdfluid conduit means for controlling fluid flow between said first andthird fluid conduit means, second valve means connected in fluidcommunication with said second and third fluid conduit means forcontrolling fluid flow between said second and third fluid conduitmeans, said first valve means including a first body connected in fluidcommunication with said first and third fluid conduit means and firstmeans responsive to a short duration decrease in the fluid pressure insaid first fluid conduit means to a fluid pressure below the fluidpressure in said third fluid conduit means while said pump is operatingin said one direction for blocking fluid flow from said this fluidconduit means into said first fluid conduit means and discharging fluidfrom said first body into said first fluid conduit means during theshort duration decrease in the fluid pressure in said first fluidconduit means while said pump is operating in said one direction and forblocking fluid flow from said first fluid conduit means into said thirdfluid conduit means and accepting fluid from said first fluid conduit atthe end of the short duration decrease in the fluid pressure in saidfirst fluid conduit means while said pump is operating in said onedirection to thereby at least partially compensate for the shortduration decrease in fluid pressure in said first fluid conduit means,said first means also being responsive to a relatively long durationdecrease in the fluid pressure in said first fluid conduit means to apressure below the fluid pressure in said third fluid conduit meanswhile said first fluid conduit means is conducting relatively lowpressure fluid from said motor to said pump during operation of saidpump in said opposite direction to enable fluid to flow from said thirdfluid conduit means to said first fluid conduit means, said second valvemeans including a second body connected in fluid communication with saidsecond and third fluid conduit means and second mans responsive to ashort duration decrease in the fluid pressure in said second fluidconduit means to a fluid pressure below the fluid pressure in said thirdfluid conduit means while said pump is operating in said oppositedirection for blocking fluid flow from said third fluid conduit means tosaid second fluid conduit means and discharging fluid from said secondbody into said second fluid conduit means during the short durationdecrease in the fluid pressure in said second fluid conduit means whilesaid pump is operating in said opposite direction, and for blockingfluid flow from said second fluid conduit means and accepting fluid fromsaid second fluid conduit means at the end of the short durationdecrease in fluid pressure in said second fluid conduit means while saidpump is operating in said opposite direction to thereby at leastpartially compensate for the short duration decrease in the fluidpressure in said second fluid conduit means, said second means alsobeing responsive to a relatively long duration decrease in the fluidpressure in said second fluid conduit means to a fluid pressure belowthe fluid pressure in said third fluid conduit means while said secondfluid conduit means is conducting relatively low pressure fluid fromsaid motor to said pump during operation of said pump in said onedirection to enable fluid to flow from said third fluid conduit means tosaid second fluid conduit means.

12. A hydrostatic transmission assembly as set forth in claim 11 whereinsaid first and second valve means each include a valve element movablymounted in a chamber defined by the body of the associated valve means,and said hydrostatic transmission assembly further includes a connectorelement operatively interconnecting said valve element of said first andsecond valve means in such a manner that when said first valve means isin an open condition enabling fluid to flow from said third fluidconduit means to said first fluid conduit means said second valve meansis in a closed condition blocking fluid flow from said third fluidconduit means to said second fluid conduit means and such that when saidsecond valve means is in an open condition enabling fluid to flow fromsaid third fluid conduit means to said second fluid conduit means, saidfirst valve means is in a closed condition blocking fluid flow from saidthird fluid conduit means to said first fluid conduit means.

13, A hydrostatic transmission assembly comprising a fluid pump, a fluidmotor, first fluid conduit means for conducting fluid between said pumpand motor upon operation of said pump to thereby operate said motorsecond fluid conduit means for connecting said first fluid conduit meansin fluid communication with a source of fluid, and valve means connectedin fluid communication with said first and second fluid conduit meansand operable between open and closed conditions under the influence ofpressure differentials of relatively long duration between said firstand second fluid conduit means to thereby control fluid flow betweensaid first and second fluid conduit means, said valve means includingmeans for retaining fluid flowing from said second fluid conduit meansinto said valve means under the: influence of a relatively shortduration variation in the pressure differential between said first andsecond fluid conduit means and for discharging the retained fluid fromsaid valve means back into said second fluid conduit means at the end ofthe relatively short duration variation in the pressure differentialbetween said first and second fluid conduit means to thereby tend tominimize the effect of relatively short duration variations in thepressure in said first fluid conduit means on the operation of saidmotor.

14, A hydrostatic transmission assembly as set forth in claim 13 whereinsaid valve means includes a valve body defining a chamber connected influid communication with said first and second fluid conduit means and amovable valve element located in said chamber, said chamber including afirst portion having a cross-sectional area which is substantially thesame as the cross-sectional area of said valve element and a secondportion having a cross-sectional area which is substantially larger thanthe cross sectional area of said valve element, said first portion ofsaid chamber and said valve element cooperating to substantially block aflow of fluid between said first and second fluid conduit means and toform a cavity which is expansible and contractable by movement of saidvalve element relative to said valve body to provide some means forretaining and discharging fluid, said second portion of said chamber andsaid valve element cooperating to enable fluid to flow between saidfirst and second fluid conduit means.

15. A hydrostatic transmission assembly as set forth in claim 13 whereinsaid means for retaining and discharging fluid includes cavity meanswhich is expansible from a contracted condition to an expanded conditionto hold fluid which flows into said valve means from said second fluidconduit means and to discharge fluid from said valve means into saidfirst fluid conduit means, said vicinity means being contractable fromthe expanded condition to the contracted condition to discharge fluidfrom said valve means into said second fluid conduit means and to enablefluid to flow from said first fluid conduit means into said valve means.

16. A hydrostatic transmission assembly as set forth in claim 13 whereinsaid valve means include s. a valve body defining a chamber having aninlet connected in fluid communication with said second fluid conduitmeans and an outlet connected in fluid communication with said firstfluid conduit means, and a valve element movable in said chamber, saidmeans for retaining and discharging fluid including an expansible cavityformed in a portion of said chamber by said valve element and connectedin fluid communication with said second fluid conduit means through saidinlet, said valve element being movable in a first direction upon theoccurrence of a relatively short duration variation in the pressuredifferential between said first and second fluid conduit means to expandsaid cavity while blocking a flow of fluid from said inlet to saidoutlet to short distance in said first direction under the influence ofa relatively short duration variation in the pressure differentialbetween said first and second fluid conduit means and is movable througha relatively large distance in said first direction under the influenceof a pressure differential of a relatively long duration between saidfirst and second fluid conduit means to thereby move said valve elementinto a position enabling fluid to flow from said inlet to said outlet.

UNITED STATES PATEiN CERTIFICATE OF CORRECTION Patent No. 3,584,460Dated June 15, 1971 Inventor(s) Keith Boyd It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

Column 8, line 4, for "vicinity" read cavity Column 9, line 22, for"this" read third column 9, line 44, for "mans" read means Column 10,line 13, insert a comma after "motor"; column 10, line 56, for"vicinity" read cavity Signed and sealed this fl th day of December 1971 (SEAL) Attest CHALK RD M.FLETCHER JR. ROBERT GOT'I? fi tiistingOfficer Acting Commissioner of Patents PC4050 USCOMM-DC suave-ps9 U 5GOVERNMENT PRINTING OFFICE: 19, :G-33l

1. A hydrostatic transmission assembly comprising a fluid pump, a fluidmotor, first fluid conduit means for conducting fluid between said pumpand motor upon operation of said pump to thereby operate said motor,second fluid conduit means for connecting said first fluid conduit meansin fluid communication with a source of fluid, and valve means connectedin fluid communication with said first and second fluid conduit meansand operable between open and closed conditions under the influence ofpressure differentials of relatively long duration between said firstand second fluid conduit means to thereby control fluid flow betweensaid first and second fluid conduit means, said valve means includingmeans for discharging fluid from said valve means into said first fluidconduit means under the influence of a relatively short durationdecrease in the pressure in said first fluid conduit means to a pressurebelow the pressure in said second fluid conduit means and for acceptingfluid from said first fluid conduit means at the end of the relativelyshort duration decrease in the pressure in said first fluid conduitmeans to thereby tend to minimize the effect of the relatively shortduration decrease in the fluid pressure in said first fluid conduitmeans.
 2. A hydrostatic transmission assembly as set forth in claim 1wherein said valve means includes a valve body defining a chamberconnected in fluid communication with said first and second fluidconduit means and a movable valve element located in said chamber, saidchamber including a first portion having a cross-sectional area which issubstantially the same as the cross-sectional area of said valve elementand a second portion having a cross-sectional area which issubstantially larger than the cross-sectional area of said valveelement, said first portion of said chamber and said valve elementcooperating to block a flow of fluid between said first and second fluidconduit means and to form a cavity which is contractable and expansibleby movement of said valve element relative to said valve body to providesaid means for discharging and accepting fluid.
 3. A hydrostatictransmission as set forth in claim 1 wherein said means for dischargingand accepting fluid includes cavity means which is contractable from afirst condition to a second condition to discharge fluid from said valvemeans into said first fluid conduit means and is expandable from saidsecond condition to said first coNdition to accept fluid from said firstfluid conduit means.
 4. A hydrostatic transmission assembly as set forthin claim 1 wherein said valve means includes a valve body defining achamber having a first portion of a relatively large cross-sectionalarea, and a valve element movable in said first portion of said chamberto provide for said discharging and accepting of fluid, said valveelement being movable from said first portion to said second portion ofsaid chamber to provide for operation of said valve means from saidclosed condition to said open condition.
 5. A hydrostatic transmissionas set forth in claim 1 wherein said valve means includes a valve bodydefining a chamber having an inlet connected in fluid communication withsaid second fluid conduit means and an outlet connected in fluidcommunication with said first fluid conduit means, a valve elementmovable in said chamber, said means for discharging and accepting fluidincluding an expansible cavity formed in a portion of said chamber bysaid valve element and connected in fluid communication with saidoutlet, said valve element being movable in a first direction upon theoccurrence of a relatively short duration decrease in the pressure insaid first fluid conduit means to a pressure below the pressure in saidsecond fluid conduit means to contract said cavity and discharge fluidfrom said valve body, said valve element being movable in a seconddirection opposite from said first direction at the end of therelatively short duration decrease in pressure to expand said cavity andaccept fluid from said first fluid conduit means.
 6. A hydrostatictransmission assembly comprising a fluid pump, a fluid motor, firstfluid conduit means for conducting fluid between said pump and motorupon operation of said pump to thereby operate said motor, second fluidconduit means for connecting said first fluid conduit means in fluidcommunication with a source of fluid, and valve means connected in fluidcommunication with said first and second fluid conduit means forcontrolling fluid flow between said first and second fluid conduitmeans, said valve means including a body connected in fluidcommunication with said first and second fluid conduit means and meansresponsive to a short duration decrease in the fluid pressure in saidfirst fluid conduit means to a fluid pressure below the fluid pressurein said second fluid conduit means for blocking fluid flow from saidsecond fluid conduit means into said first fluid conduit means anddischarging fluid from said body into said first fluid conduit meansduring the short duration decrease in fluid pressure in said first fluidconduit means and for blocking fluid flow from said first fluid conduitmeans into said second fluid conduit means and accepting fluid from saidfirst fluid conduit means at the end of the short duration decrease inthe fluid pressure in said first fluid conduit means to thereby at leastpartially compensate for the short duration decrease in fluid pressurein said first fluid conduit means, said means responsive to a shortduration decrease in the fluid pressure in said first fluid conduitmeans also being responsive to a relatively long duration decrease inthe fluid pressure in said first fluid conduit means to a fluid pressurebelow the fluid pressure in said second fluid conduit means to enablefluid to flow from said second fluid conduit means to said first fluidconduit means.
 7. A hydrostatic transmission assembly as set forth inclaim 6 wherein said means responsive to a short duration decrease inthe fluid pressure in said first fluid conduit means includes a movablevalve element for blocking fluid flow between said first and secondfluid conduit means, said valve element being located in a chamberdefined by said body and cooperating with said body to define a cavityin said chamber, said valve element being movable in a first directionunder the influence of the pressure differential between said first andsecond fluid condUit means during the short duration decrease in fluidpressure in said first fluid conduit means to contract said cavity andthereby discharge fluid from said vicinity into said first fluid conduitmeans, said valve element being movable in a second direction oppositeto said first direction under the influence of a pressure differentialbetween the first and second fluid conduit means to expand said cavityand to accept therein fluid from said first fluid conduit means at theend of the short duration decrease in fluid pressure in said first fluidconduit means, said valve element cooperating with said valve bodyduring the short duration decrease in fluid pressure in said first fluidconduit means to block fluid flow between said first and second fluidconduit means.
 8. A hydrostatic transmission assembly as set forth inclaim 7 wherein said valve element is movable for a relatively largedistance in said first direction under the influence of a relativelylong duration decrease in the fluid pressure in said first fluid conduitmeans to a fluid pressure below the fluid pressure in said second fluidconduit means to move said valve element to a position in which itenables fluid to flow from said second fluid conduit means to said firstfluid conduit means.
 9. A hydrostatic transmission assembly as set forthin claim 7 wherein said valve element cooperates with said body todefine another cavity in said chamber, said other cavity being connectedin fluid communication with said second fluid conduit means and beingexpanded by movement of said valve element in said first directionduring the sort duration decrease in fluid pressure in said first fluidconduit means to accept in said other cavity fluid from said secondfluid conduit means during the short duration decrease in fluid pressurein said first fluid conduit means, said other cavity being contracted bymovement of said valve element in said second duration at the end of theshort duration decrease in fluid pressure in said first fluid conduitmeans to thereby discharge accepted fluid from said second fluid conduitmeans back into said second fluid conduit means.
 10. A hydrostatictransmission assembly comprising a fluid pump, a fluid motor, firstfluid conduit mean for conducting fluid between said pump and motor,said motor means having an inherent displacement variation during lowspeed operation resulting in a short duration pressure decrease in saidfirst fluid conduit means, second fluid conduit means for connectingsaid first fluid conduit means in fluid communication with source offluid, and compensating means including a body connected in fluidcommunication with said first and second fluid conduit means, saidcompensating means being responsive to the short duration decrease inthe fluid pressure in said first fluid conduit means to a fluid pressurebelow the fluid pressure in said second fluid conduit means fordischarging fluid from said body into said first fluid conduit meansduring the short duration decrease in fluid pressure in said first fluidconduit means and for accepting fluid from said first fluid conduitmeans at the end of the short duration decrease in the fluid pressure insaid first fluid conduit means to thereby at least partially compensatefor the short duration decrease in fluid pressure in said first fluidconduit means, said compensating means including a movable elementlocated in a chamber defined by said body and cooperating with said bodyto define a cavity in said chamber, said element being movable in afirst direction under the influence of the short duration decrease influid pressure in said first fluid conduit means to contract said cavityand thereby discharge fluid from said cavity into said first fluidconduit means, said element being movable in a second direction oppositeto said first direction under the influence of the fluid pressure insaid first fluid conduit means to expand said cavity and to accepttherein fluid from said first fluid conduit means at the end Of theshort duration decrease in fluid pressure in said first fluid conduitmeans.
 11. A hydrostatic transmission assembly comprising a reversiblefluid pump, a reversible fluid motor, first fluid conduit means forconducting high pressure fluid from said pump to said motor duringoperation of said pump in one direction and for conducting relativelylow pressure fluid from said motor to said pump during operation of saidpump in an opposite direction, second fluid conduit means for conductingrelatively low pressure fluid from said motor to said pump duringoperation of said pump in said one direction and for conductingrelatively high pressure fluid from said pump to said motor duringoperation of said pump in said opposite direction, third fluid conduitmeans for connecting said first and second fluid conduit means in fluidcommunication with a relatively low pressure source of fluid, firstvalve means connected in fluid communication with said first and thirdfluid conduit means for controlling fluid flow between said first andthird fluid conduit means, second valve means connected in fluidcommunication with said second and third fluid conduit means forcontrolling fluid flow between said second and third fluid conduitmeans, said first valve means including a first body connected in fluidcommunication with said first and third fluid conduit means and firstmeans responsive to a short duration decrease in the fluid pressure insaid first fluid conduit means to a fluid pressure below the fluidpressure in said third fluid conduit means while said pump is operatingin said one direction for blocking fluid flow from said this fluidconduit means into said first fluid conduit means and discharging fluidfrom said first body into said first fluid conduit means during theshort duration decrease in the fluid pressure in said first fluidconduit means while said pump is operating in said one direction and forblocking fluid flow from said first fluid conduit means into said thirdfluid conduit means and accepting fluid from said first fluid conduit atthe end of the short duration decrease in the fluid pressure in saidfirst fluid conduit means while said pump is operating in said onedirection to thereby at least partially compensate for the shortduration decrease in fluid pressure in said first fluid conduit means,said first means also being responsive to a relatively long durationdecrease in the fluid pressure in said first fluid conduit means to apressure below the fluid pressure in said third fluid conduit meanswhile said first fluid conduit means is conducting relatively lowpressure fluid from said motor to said pump during operation of saidpump in said opposite direction to enable fluid to flow from said thirdfluid conduit means to said first fluid conduit means, said second valvemeans including a second body connected in fluid communication with saidsecond and third fluid conduit means and second mans responsive to ashort duration decrease in the fluid pressure in said second fluidconduit means to a fluid pressure below the fluid pressure in said thirdfluid conduit means while said pump is operating in said oppositedirection for blocking fluid flow from said third fluid conduit means tosaid second fluid conduit means and discharging fluid from said secondbody into said second fluid conduit means during the short durationdecrease in the fluid pressure in said second fluid conduit means whilesaid pump is operating in said opposite direction, and for blockingfluid flow from said second fluid conduit means and accepting fluid fromsaid second fluid conduit means at the end of the short durationdecrease in fluid pressure in said second fluid conduit means while saidpump is operating in said opposite direction to thereby at leastpartially compensate for the short duration decrease in the fluidpressure in said second fluid conduit means, said second means alsobeing responsive to a relatively long duration decrease in the fluidpressure in said secOnd fluid conduit means to a fluid pressure belowthe fluid pressure in said third fluid conduit means while said secondfluid conduit means is conducting relatively low pressure fluid fromsaid motor to said pump during operation of said pump in said onedirection to enable fluid to flow from said third fluid conduit means tosaid second fluid conduit means.
 12. A hydrostatic transmission assemblyas set forth in claim 11 wherein said first and second valve means eachinclude a valve element movably mounted in a chamber defined by the bodyof the associated valve means, and said hydrostatic transmissionassembly further includes a connector element operativelyinterconnecting said valve element of said first and second valve meansin such a manner that when said first valve means is in an opencondition enabling fluid to flow from said third fluid conduit means tosaid first fluid conduit means said second valve means is in a closedcondition blocking fluid flow from said third fluid conduit means tosaid second fluid conduit means and such that when said second valvemeans is in an open condition enabling fluid to flow from said thirdfluid conduit means to said second fluid conduit means, said first valvemeans is in a closed condition blocking fluid flow from said third fluidconduit means to said first fluid conduit means.
 13. A hydrostatictransmission assembly comprising a fluid pump, a fluid motor, firstfluid conduit means for conducting fluid between said pump and motorupon operation of said pump to thereby operate said motor second fluidconduit means for connecting said first fluid conduit means in fluidcommunication with a source of fluid, and valve means connected in fluidcommunication with said first and second fluid conduit means andoperable between open and closed conditions under the influence ofpressure differentials of relatively long duration between said firstand second fluid conduit means to thereby control fluid flow betweensaid first and second fluid conduit means, said valve means includingmeans for retaining fluid flowing from said second fluid conduit meansinto said valve means under the influence of a relatively short durationvariation in the pressure differential between said first and secondfluid conduit means and for discharging the retained fluid from saidvalve means back into said second fluid conduit means at the end of therelatively short duration variation in the pressure differential betweensaid first and second fluid conduit means to thereby tend to minimizethe effect of relatively short duration variations in the pressure insaid first fluid conduit means on the operation of said motor.
 14. Ahydrostatic transmission assembly as set forth in claim 13 wherein saidvalve means includes a valve body defining a chamber connected in fluidcommunication with said first and second fluid conduit means and amovable valve element located in said chamber, said chamber including afirst portion having a cross-sectional area which is substantially thesame as the cross-sectional area of said valve element and a secondportion having a cross-sectional area which is substantially larger thanthe cross sectional area of said valve element, said first portion ofsaid chamber and said valve element cooperating to substantially block aflow of fluid between said first and second fluid conduit means and toform a cavity which is expansible and contractable by movement of saidvalve element relative to said valve body to provide some means forretaining and discharging fluid, said second portion of said chamber andsaid valve element cooperating to enable fluid to flow between saidfirst and second fluid conduit means.
 15. A hydrostatic transmissionassembly as set forth in claim 13 wherein said means for retaining anddischarging fluid includes cavity means which is expansible from acontracted condition to an expanded condition to hold fluid which flowsinto said valve means from said second fluid conduit means and todischargE fluid from said valve means into said first fluid conduitmeans, said vicinity means being contractable from the expandedcondition to the contracted condition to discharge fluid from said valvemeans into said second fluid conduit means and to enable fluid to flowfrom said first fluid conduit means into said valve means.
 16. Ahydrostatic transmission assembly as set forth in claim 13 wherein saidvalve means include s a valve body defining a chamber having an inletconnected in fluid communication with said second fluid conduit meansand an outlet connected in fluid communication with said first fluidconduit means, and a valve element movable in said chamber, said meansfor retaining and discharging fluid including an expansible cavityformed in a portion of said chamber by said valve element and connectedin fluid communication with said second fluid conduit means through saidinlet, said valve element being movable in a first direction upon theoccurrence of a relatively short duration variation in the pressuredifferential between said first and second fluid conduit means to expandsaid cavity while blocking a flow of fluid from said inlet to saidoutlet to thereby retain fluid flowing into said valve body from saidsecond fluid conduit means, said valve element being movable in a seconddirection opposite from said first direction at the end of therelatively short duration variation in the pressure differential betweensaid first and second fluid conduit means to contract said cavity whileblocking a flow of fluid from said inlet to said outlet to therebydischarge retained fluid from said cavity into said second fluid conduitmeans.
 17. A hydrostatic transmission assembly as set forth in claim 16wherein said valve element is movable through a relatively shortdistance in said first direction under the influence of a relativelyshort duration variation in the pressure differential between said firstand second fluid conduit means and is movable through a relatively largedistance in said first direction under the influence of a pressuredifferential of a relatively long duration between said first and secondfluid conduit means to thereby move said valve element into a positionenabling fluid to flow from said inlet to said outlet.